The FBI have now said that since their appeal was launched, it has been contacted by "several hundred individuals from around the globe wishing either to reach out as potential victims or provide information in the ongoing investigation".

Authorities fear Vahey could prove to be one of the most prolific paedophiles in recent history. Many of his victims may not even realise they were molested, as it is thought he drugged his victims with sleeping pills before abusing them.

Questions have been asked how Vahey was able to go undetected for so many years. In 1969, he was convicted of molesting children at a high school in California, where he taught swimming.

Despite being sentenced to 90 days in jail and required to sign on California's sex offenders' registry, he was able to take up his first teaching job at a school in Iran in 1972.

From there, he went on to take up a number of teaching roles in countries like Venezuela, Spain and Saudi Arabia.

When confronted about how a convicted paedophile was able to get a job at Southbank International School, Woodhead said the £25,000-a-year school in Westminster had conducted a CRB check on Vahey and examined his employment history for the previous 17 years.

He added he registered to be a teacher in 1986 and assumed he would not have any prior convictions to be able to do so.

Jane Larsson, executive director of the Council of International Schools, said a group of six international education associations are examining ways to close loopholes which allow paedophiles to move from country to country without being properly checked.

"When this kind of thing happens it's a shock to everyone and it mobilises action," she said.